1. Field of the Invention
The present invention relates to an illumination optical system to be used for providing increased service efficiency of light and a sharp image, and to a projector including the illumination optical system.
2. Description of the Related Art
FIG. 9 is a perspective external view illustrating a typical projector. Referring to FIG. 9, a rectangular parallelepiped projector 501 includes an upper case 503 which defines the upper surface thereof and is provided with operating buttons 502, a lower case 504 which defines the lower surface of the projector 501, and a front case 505 which defines the front surface of the projector 501. There is a projection lens 506, the front-end portion of which protrudes from the front case 505.
For example, such a projector as mentioned above includes a known optical system configured as shown in FIG. 10.
That is, the projector 501 includes a light source 510, and an illumination optical system 520 for providing a uniform illumination distribution of light emitted from the light source 510 and for allowing the light to impinge upon liquid crystal panels 550R, 550G, 550B with the same polarization. The projector 501 also includes a color beam splitting optical system 530 for splitting a beam W launched from the illumination optical system 520 into red, green, and blue beams R, G, B, and a relay optical system 540 for introducing the blue beam B in the beams R, G, B, which have been split by the color beam splitting optical system 530, into the liquid crystal panel 550B associated with the blue beam B. The projector 501 further includes the three liquid crystal panels 550R, 550G, 550B as light modulating means for modulating each color beam in accordance with given image information, a cross dichroic prism 560 as a color beam combining optical system for combining modulated respective color beams, and the projection lens 506 for expanding combined beams and projecting the beams onto a projection screen.
FIG. 11 is a schematic view illustrating the action of the illumination optical system 520. As shown in FIG. 11, the illumination optical system 520 allows a first lens array 521 to split the light emitted from the light source 510 into a plurality of sub-beams, then the sub-beams to impinge upon a polarization conversion element array 523 via a second lens array 522, and then the polarization conversion element array 523 to provide the respective sub-beams with the same polarization direction. Thereafter, the illumination optical system 520 allows a superimposing lens 524 to superimpose the sub-beams on the image forming area of the liquid crystal panels 550R, 550G, 550B.
The illumination optical system 520 functions as described above to allow polarized light of one type to illuminate uniformly all parts of the respective liquid crystal panels 550R, 550G, 550B. Therefore, when displaying an image with a projector, the system contributes to providing a sharp and high contrast image all over the display area.
Below, the process of providing the same polarization direction (of converting polarization) in the aforementioned illumination optical system 520 will be explained in more detail with reference to FIGS. 11 and 12. FIG. 11 is an enlarged view illustrating part of the illumination optical system 520. The polarization conversion element array 523 is configured to have a polarization splitting film 523b and a reflective film 523c, which are alternately disposed between a plurality of light transmissive members 523a. There is also provided a half-wave plate 523d on the transmission side corresponding to the polarization splitting film 523b. The first lens array 521 splits the light emitted from the light source 510 into a plurality of sub-beams and then condenses the respective sub-beams near the polarization splitting film 523b of the polarization conversion element array 523. Accordingly, condensed images of the respective sub-beams are formed near the polarization splitting film 523b. The condensed images are derived from the light-emitting portion of the light source 510. The incident light upon the polarization splitting film 523b is split into p- and s-polarized beams, and the p-polarized beam which is a transmitted beam is converted into an s-polarized beam with the half-wave plate 523d. On the other hand, the s-polarized beam which is a reflected beam is directed substantially in the same direction as the p-polarized beam at the reflective film 523c and then launched from the polarization conversion element array 523 without any change. As described above, the respective sub-beams that have been split with the first lens array are provided with the same polarization. To achieve such an ideal polarization conversion, it is necessary to allow the incident light upon the polarization conversion element array 523 to impinge only upon the polarization splitting film 523b. This is because incidence of light upon the reflective film 523c would cause the light to be converted reversely in polarization direction. Incidentally, FIG. 11 shows the incidence portion corresponding to the polarization splitting film 523b as an effective incidence portion 523e. 
Now, suppose the light-emitting portion of the light source 510 is an ideal point light source and the illumination optical system 520 is an ideal one that has no errors in its design and fabrication. In this case, a condensed point image would be formed near the polarization splitting film 523b. It would be therefore possible in this case to allow the incident light upon the polarization conversion element array 523 to impinge only upon the polarization splitting film 523b. However, in practice, the light-emitting portion of the light source 510 has a given size, thereby providing a given size to the condensed image formed near the polarization splitting film 523b. The size of the condensed image often exceeds that of the effective incident portion 523e in the ideal system. In such a case, as schematically shown in FIG. 12, the effective incident portion 523e may be made larger than that of the ideal system in order to allow the light to impinge only upon the effective incident portion 523e. At this time, it is necessary for the first lens array 521 to direct the sub-beams outwardly except for the central portion thereof. In some cases, some lenses constituting the first lens array 521 are decentered (i.e., the optical axis of the lenses is shifted from their geometric center).
The lenses that constitute the first lens array and are decentered for service have step differences at their boundaries due to differences in shape of their surfaces. The presence of a step difference often produces a portion with a rounded edge (i.e., the edge on the periphery of a lens is not formed at a specified angle but in a curved shape) during the manufacture of the lens array. Incident light upon the rounded edge cannot reach an illuminated area or the image forming area of the liquid crystal panels 550R, 550G, 550B, so that the peripheral portion of the illuminated area becomes dark. Consequently, the portion corresponding to the rounded edge appears as a display shadow in the projected area as shown in FIG. 13 when projecting the light onto the screen, conventionally, it has been obliged to provide an excess illumination margin at the expense of brightness in order to avoid the display shadow.
The present invention was developed to solve the aforementioned problems without sacrificing brightness, employing the following configurations.
An illumination optical system according to the present invention includes a lens array with a plurality of lenses for splitting light emitted from a light source into a plurality of sub-beams, at least some of the lenses of the lens array being decentered. The illumination optical system is characterized in that the thickness of each of the lenses constituting the lens array is adjusted such that a step difference at boundaries between the lenses falls within a predetermined step difference. For example, the predetermined step difference is such as to prevent a rounded edge, caused by the step difference, from occurring at the boundaries between the lenses upon manufacture of the lens array. This illumination optical system makes it possible to provide bright illumination and a reduced illumination margin. Therefore, the use of the illumination optical system for a projector makes it possible to prevent or suppress display shadows without sacrificing brightness when displaying an image.
Here, in the case where each of the lenses constituting the lens array is decentered for each column of lenses, it is preferable that the thickness of each lens is adjusted for each column. This makes it possible to reduce the step difference at boundaries of the lenses in each column.
In addition, in the case where each of the lenses constituting the lens array is decentered for each lens, it is preferable that the thickness of a lens is adjusted for each lens to make d/S equal to or less than a predetermined value where S is a vertical size x a horizontal size of each of the lenses and d is the step difference at the boundaries between the lenses. This makes it possible to reduce the step difference at all lens boundaries.
Furthermore, the illumination optical system preferably includes a polarization conversion element for providing the same polarization for each of the sub-beams split by the lens array. The system also preferably includes a superimposing lens for superimposing the sub-beams provided with the same polarization direction by the polarization conversion element on a predetermined position. This makes it possible to provide further increased service efficiency of light.
On the other hand, the projector according to the present invention includes an electro-optical device and employs the illumination optical system in which the thickness of the lenses constituting the lens array is adjusted as described above.
The projector according to the present invention may further include a color beam splitting optical system for splitting the light beams emitted by the illumination optical system into beams of three colors. The projector may also include a plurality of the electro-optical devices for modulating each of the color beams split by the color beam splitting optical system, a color beam combining optical system for combining modulated beams of each color, and a projection lens for projecting a combined beam.
These projectors can employ the illumination optical systems described above, thereby making it possible to prevent or suppress display shadows without sacrificing brightness.